Pore and fracture characteristics of low-maturity continental shale and its significance for shale oil occurrence: A case study of Shahejie Formation in Qingnan Sag, Jiyang Depression

Low-maturity shale oil has great resource potential in Jiyang Depression and has become an important field after the successful breakthrough of medium- to high-maturity shale oil. Objective The purpose of this paper is to clarify the pore and fracture characteristics of low-maturity continental shale and their significance to shale oil occurrence. Methods By studying the Es3x and Es4scs members in Qingnan Depression, Jiyang Depression, this study combines thin section observation, TOC content, XRD analysis, solvent extraction, low-temperature N2 adsorption, high-pressure mercury injection, scanning electron microscope observation and EDS analysis to classify the shale lithofacies, characterize the pore and fracture development, and illustrate the importance of shale pores and fractures for shale oil occurrence. Results The TOC content is mostly in the range of 1.0%-4.0%, and the mineral composition mainly includes felsic minerals, followed by clay and carbonate minerals. The major pore types are ink-bottle-shaped pores and plate-parallel-shaped pores, including quartz intergranular pores, clay mineral interlamellar pores, and dolomite intercrystalline pores. The major pore sizes are mostly less than 200 nm and exhibit a multipeak distribution, which is concentrated in the ranges of 2-50 nm, 50-80 nm and 100-200 nm. Horizontal bedding fractures, vertical fractures and network fractures developed in the studied shales and were mostly filled or contacted by bitumen.Felsic-rich shale lithofacies usually have a greater pore volume and specific surface area than clay-rich lithofacies, and felsic minerals positively contribute to pore development. The pore volume and specific surface area first decreased and then increased with increasing thermal maturity. When Ro>0.6%, the shale oil content increases significantly, which is mainly related to the large amount of hydrocarbon generation in organic matters. Conclusion Horizontal bedding fractures, quartz intergranular pores, and dolomite and calcite intercrystalline pores are favourable storage and occurrence spaces for shale oil.